Currently, when a user equipment (UE) needs and has an ability of carrier aggregation (CA), a base station considers the ability of CA of the UE and selects available carriers for the UE, and configures a cell as a secondary cell (SCell) of the UE via a radio resource control (RRC) message.
For the sake of punctuality, if it is deemed that the configured SCell may immediately be used for data transmission, then the SCell needs to be cancelled, and the UE does not use resource of the SCell to transmit data any longer when the SCell is not used considering the resource efficiency; and when the cell is again needed for data transmission, the cell is again needed to be configured as an SCell via an RRC message.
When a type of traffic occurring frequently of a short duration and a large traffic amount is taken into account, frequently configuring/de-configuring an SCell in such a way will consume a large amount of RRC messages; and at the same time, punctuality of an RRC message is relatively poor, and if an SCell is configured via an RRC message when needed, it is possible that timeliness of such a traffic cannot be satisfied. On the contrary, if the cell is always taken as an SCell so as to save RRC messages and ensure timeliness of a traffic, even if there exists no traffic for transmission, resources cannot be fully used, spectral utilization is low, and a throughput of the cell drops therewith.
Thus, a new state is introduced for an SCell: an activation state or a deactivation state. UE only monitors an activated SCell, and transmits data only in the activated SCell. When a base station configures CA UE with an available SCell, both the base station and the UE default that the SCell is in a deactivation state. When needed, the base station sets the SCell to be in an activation state via an activation media access control (MAC) control element (CE); otherwise, the base station sets the SCell to be in a deactivation state via a deactivation MAC CE.
In some cases, it is possible that a transmission failure of a deactivation MAC CE deactivating an SCell occurs, hence, the base station will deem that the SCell is in a deactivation state, and the UE will deem that the SCell is in an activation state; and the UE may perform autonomous uplink transmission, such as a sounding reference signal (SRS), on the SCell; which may pose interference on other uplink transmission, and is disadvantageous to energy-saving. Therefore, an UE autonomous deactivation mechanism is introduced.
That is, in order to unify the state of an SCell activation/deactivation by the UE and the base station, an UE autonomous deactivation mechanism is introduced, namely, a deactivation timer is maintained for each SCell. When a base station activates an SCell via an activation MAC CE, both the base station and the UE deem that the SCell is activated, and start a deactivation timer for the SCell; and when a deactivation MAC CE is successfully transmitted or the deactivation timer associated with the SCell expires, both the UE and the base station deem that the SCell is deactivated.
If a physical downlink control channel (PDCCH) indicating an uplink grant or downlink assignment is received in the activated SCell, or a PDCCH scheduling the SCell, including an uplink grant and downlink assignment, is received in another serving cell, the UE restarts the deactivation timer associated with the SCell.
Based on a demand of the UE for traffics, if an SCell is not needed temporarily, the base station may deactivate the SCell; and even if the UE does not receive a deactivation MAC CE, only if the base station does not schedule the SCell and not transmit a PDCCH in the SCell within a period of time, and the SCell can be autonomously deactivated by the UE via the deactivation timer.
Based on the UE autonomous deactivation mechanism, the activation state of the SCell of the CA UE may be appropriately maintained. Although both the base station and the UE maintain a deactivation timer for each SCell, values of these timers are identical, which may be provided by the base station via the RRC message, and the default value of the timers is an infinite value.
On the other hand, in an existing LTE system, for CA UE, only a primary cell (PCell) is configured with one or more physical uplink control channels (PUCCHs), and almost all uplink control information (UCI) in all cells is transmitted to a base station (such as an eNB) via the PUCCHs of the PCell.
As occurrence of a demand for new traffics, the number of downlink carriers of CA becomes larger and larger, and the number of UE (including CA UE and non-CA UE) becomes larger and larger, problems will occur when UCI is only transmitted in a PCell. In order to offload for transmitting UCI, in addition to transmitting PUCCHs in the PCell, a base station may further select an SCell for UE having an ability of UL CA, which is used for transmitting PUCCHs. Thus, UCI related to the PCell is transmitted in the PCell, UCI of an SCell carrying PUCCHs is transmitted in the SCell, and UCI of other SCells is configured by the base station to be transmitted in the PCell or is configured to be transmitted in the SCell carrying PUCCHs.
That is, in a current CA, there exist two serving cells carrying PUCCHs, one is the PCell, and the other is the SCell carrying PUCCHs. The PCell and all normal SCells whose UCI is transmitted via the PUCCHs of the PCell constitute a primary PUCCH cell group, and the SCell carrying PUCCHs and all normal SCells whose UCI is transmitted via the PUCCHs of the SCell constitute a secondary PUCCH cell group. The primary PUCCH cell group and secondary PUCCH cell group are collectively referred to as a PUCCH cell group.
It should be appreciated that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of this disclosure.